(1) Field of the Invention
The present invention relates to an expandable shaft with an expandable external dimension to adjust the fit between the shaft and an element surrounding the shaft. The present invention is particularly useful in the context of a bicycle hub.
(2) Description of the Related Art
The current state of the art in bicycle hub construction utilizes two conventional deep-groove radial cartridge bearing assemblies. This type of bearing is very sensitive to axial misalignment between the outer race and the inner race. Any misalignment will cause premature wear and a rough rotating action of the bearing assembly.
The current state of the art hub shell commonly includes a hub shell, two bearing assemblies, an axle with a pair of axle caps at its ends. The axle commonly includes a pair of axially spaced shoulders to axially locate and separate the inner race of these two bearing assemblies. Due to the inevitable tolerances inherent in the manufacture of the axle and hub shell components, the axial spacing of the axle shoulders are rarely, if ever, are perfectly matched to the axial spacing of the hub shell shoulders. This means that the bearing assemblies cannot be made to perform optimally on a consistent and reliable basis.
With this conventional arrangement, the axle, which is usually turned on a lathe is a relatively expensive component to fabricate: firstly because the starting stock diameter needs to be oversized to accommodate the outside diameter of the shoulders, secondly because of the machining time to remove this excess stock in the non-shoulder areas, thirdly due to the accurate axial spacing of the shoulders required in the attempt to minimize any misalignment of the bearings, and fourthly because the diameter of the axle must be very accurately controlled in the attempt to create a controlled fit between the outside diameter of the axle and the inside diameter of the bearing.
The fit of the outside diameter of the axle and the inside diameter of the mating bearing assembly must be very closely controlled such that sufficient clearance exists to permit assembly without applying excessive axial load on the bearing, while at the same time reducing this clearance in an attempt to minimize any looseness or free-play between the two. The optimum fit is very difficult, if not impossible to achieve and usually results in some degree of free-play in the system.
Although some attempts have been made to employ a spring washer into this conventional hub design to create a preload to the bearing assembly, this has only been partially successful. Firstly, in order to permit the preload spring washer to apply a preload, there needs to be sufficient clearance between the axle and the bearing to allow the inner race to be axially displaced by the spring washer. This clearance obviously adds to the undesirable free-play of the system. Secondly, since the spring washer is a yieldable element, the inner race of the bearing may now shift relative to the axle, any axial loads applied to the rim of the wheel will tend to compress this spring washer and allow the hub shell to axially shift relative to the axle. This can impart a wobbly or flexy feel to the wheel, resulting in a diminished feeling of control on the part of the rider.